Sodium polysulfide pulping process and regeneration



United States Patent 3,470,061 SODIUM POLYSULFIDE PULPING PROCESS AND REGENERATION Richard G. Barker, Princeton Junction, N.J., assignor to Union Camp Corporation, New York, N.Y., a corporation of Virginia No Drawing. Filed Mar. 3, 1967, Ser. No. 620,288

Int. Cl. D21c 11/12, 3/26 US. Cl. 162--32 Claims ABSTRACT OF THE DISCLOSURE A method for generating sodium polysulfide in alkaline digesting liquor containing sodium sulfide comprising chemically oxidizing the sodium sulfide present in the digesting liquor with insoluble solid inorganic manganese oxidizer, the oxidizer being an oxide, sulfide or hydroxide exemplified by the formulae Mn O Mn O MnS partially oxidized MnO, MnOOH and MnO the oxidation occurring at a temperature in the range of 40120 C.

The present invention relates to a novel method for the manufacture of paper pulp by a sodium polysulfide pulping process and, particularly, to the novel means for generating sodium polysulfide from sodium sulfide pulping liquors by employing an insoluble, oxidizing manganese compound resulting in great savings in the cost of pulp manufacture. These savings are based on higher product yields from a given amount of wood, and on a more efiicient and more economical means of generating polysulfide than is presently available. The spent manganese oxidant of this invention is also insoluble to simplify its separation from the pulping liquor and thereby to make the polysulfide process truly regenerative and more economical on a cyclic basis. The inorganic, insoluble manganese oxidant compound is preferably an oxide, sulfide or hydroxide, such as exemplified by the formulae Mn O Mn O MnS partially oxidized MnO, MnOOH and MnO All of these typical oxidants contain manganese in a higher valence state to be effective as oxidizing agents in converting sodium sulfide to sodium polysulfide at a temperature of 40 C. to 120 C. The oxidation is visualized as taking place by the following exemplary reactions:

More particularly, this invention relates to a method of preparing a new sodium polysulfide cooking liquor which is particularly efficient for pulp digestion and paper manufacture and which results from chemically oxidized starting solution (as exemplified by kraft white liquor, green liquor, or sodium sulfide) in the presence of an insoluble inorganic manganese oxidant compound. This oxidation provides a concentration of sodium polysulfide which ranges widely from about 2 to 45 grams per liter as polysulfide sulfur and corresponds to an initial sodium sulfide concentration of 13 grams per liter up to 225 grams per liter.

The present alkaline oxidation is easily adapted to be used in the conventional kraft pulping process Where sulfidity values between 15 and 35% are generally employed by virtue of the fact that the' novel oxidation step employs an oxidizing insoluble manganese compound, which produces the desired oxidation of sodium sulfide in alkaline liquor to form sodium polysulfide, whereafter all of the spent oxidant together with any unused oxidizing agent, is recovered in insoluble form. No losses occur because the reduced manganese compound is largely insoluble in the alkaline liquor and can be physically separated from the alkaline sulfide liquor by ordinary filterinf means, such as a screen, a filter, by decanting, or the After separation in a generating part of the system, the oxidized alkaline liquor contains the polysulfide in a concentration of from about 2 to about 45 grams per liter which is adapted for various types of woods to be pulped. The reduced insoluble manganese compound which constitutes the spent solid oxidant is then subjected to a separate oxidation treatment in a regeneration part of the system. Regeneration is carried out with air or oxygen to convert spent insoluble oxidant to a higher oxidation state of the manganese compound so that it can be used again as the insoluble oxidant for the generation step.

The outstanding advantages of the present alkaline polysulfide process in paper manufacture are first, the complete adaptability without extensive equipment change for the conventional kraft process, a simple oxidizing unit being inserted in the line; and second, the nature of the oxidation process, which results in no change in the chemical composition of the liquor after it has been recovered by the conventional kraft recovery process. The efficient recovery of the kraft process, e.g., a recovery greater than of the chemicals from the residual spent cooking liquor, would be unchanged and the sulfursoda balance of the system would be unchanged. After conventional recovery, the recovered chemicals can be reused for further polysulfide conversion in the next cyclic operation without the' need for removing sodium sulfide and adding sulfur or polysulfide, as has been suggested by the prior art of polysulfide pulping.

Although the present invention is particularly adaptable to the kraft process, it is also applicable for other pulping liquors, such as sodium sulfide and hydrosulfide.

The benefits of polysulfide pulping in paper manufacture are known but these benefits have not been achieved because there has been no simple, low-cost method for creating the desired sodium polysulfide concentration from initial sodium sulfide containing liquors. The prior art proposals have been either too costly because of higher sulfur and alkali requirements, or too complex in the requirement for complicated apparatus for the recovery of sulfur and generation of polysulfide which needs to be manned by highly skilled personnel.

Also, recovery procedures heretofore have not been satisfactory. As stated in Landmark US. Patent No. 3,216,887, the fortification of cooking liquor with sulfur or sodium polysulfide addition interferes with the recovery of both sulfur and alkali in the liquor recovery system. Although the air oxidation of black liquor-white liquor mixtures is preferred by Landmark over conventional sulfur addition, it does contribute to excessive sulfur losses in the recovery due to the high sulfidity requirement. This high sulfidity results in increased corrosion and air pollution during chmeical recovery of the spent liquor. These disadvantages are thoroughly documented in the technical and patent literature. The present invention eliminates these disadvantages by employing a novel apparatus to oxidize the conventional kraft liquor with no change in sulfidity.

Another proposal to gain the known benefit of increased paper yield in sodium polysulfide pulping is found in the US. Patent of Ferrigan, No. 3,210,235, who suggests a controlled sulfur replenishment to the alkaline liquor employed in the kraft cooking system. However, the sulfur additions in Ferrigan are larger than the total amounts of sulfur which are normally required for replenishment in the kraft digestion system, thereby increasing the material cost. This excess amount of sulfur appears in the form of a larger sodium sulfide content after the combustion stage to increase the difficulty and the cost of recovery due to the corrosive nature of the sulfide and excessive sulfur losses to the atmosphere. Ferrigan suggests, in an effort to offset these costs, that a fraction of the green liquor, equivalent to 55-65% of the total, be treated in a separate manner which is entirely different than the usual kraft fortification procedure. However, this separate procedure requires special equipment and thereby results in increased equipment and operating costs in order to recover this sodium sulfide as elemental sulfur.

Kibrick US. Patent No. 2,944,928 also describes the improvement in yield by the fortification of the kraft cooking liquor with sodium polysulfide, but, like Ferrigan, provides no teaching of an economical recovery process which can overcome the problems of equipment corrosion, sulfur losses and increased costs encountered by Ferrigan.

The oxidation process of the present invention avoids the addition of sulfur and polyuslfide, in accordance with the teachings of Ferrigan and Kibrick, but oxidizes the sulfide ion already present in the cooking liquor to polysulfide and, therefore, does not produce a substantially increased content of sodium sulfide which would increase sulfidity, as in Ferrigan and Kibrick, to render the process uneconomical. Uniquely, the present process oxidizes sodium sulfide with a solid inorganic material but does not require any high sulfidity, as in the Landmark patent, and is uniquely effective at both low and high sulfidities. Further, the present process does not require the addition of black liquor, as does the Landmark patent; yet it works very efficiently under conditions of dilution, employing black liquor as the diluent for white liquor prior to oxidation.

As pointed out above, the advantages of the present process over the process of Landmark lie in the independence of the need to mix special white and black liquors for aeration and oxidation to avoil foaming created by the black liquor, in the elimination of the high sulfidity requirement in Landmark, and in the greater efiiciency of polysulfide production. Although the present process does not require high sulfidity to work, it is also effective at high sulfidity.

The following table compares the manganese dioxide oxidation of white liquor with the air oxidation of white liquor-black liquor mixtures. Air oxidations in Columns A and B in the table were carried out in the laboratory, while C and D are the two examples given in the Landmark patent. The greater efliciency of the maganese dioxide process is evident.

TABLE I Air Oxidation A B C D Mixed Mixed Mixed Mixed Process H1102 Liquor Liquor Liquor Liquor Ratio white to black liquor 1:1 1:1 1:6:1 1:621 Initial NazS, g.p.l 80 41.8 48 73. 1 46 sulfidity, percent- 60 60 60 71 50 Oxidation time (minutes)--. 20 60 300 25 5 Temperature C.) 60 70 25 70 70 Percent Nays oxidized 54 31 38. 6 49. 4 45. 7 Polysuliide sulfur, g.p.l 14 1. 4 2. 65 7. 27 4. 37 Efficiency (percent) 78 26. 5 25 45. 2 50. 6

Although the optimum conditions for chemical oxidation of sodium sulfide in alkaline solution with the inorganic manganese insoluble compound of the invention may depend upon the size of equipment, scale of regeneration, production requirements and continuous versus around the clock batch considerations, in general, about 1 mole part of manganese compound expressed as manganese dioxide, MnO per mole of sodium sulfide is effective to convert the sulfide to polysulfide in from 2 minutes at 120 C. down to 120 minutes at 40 C. In most commercial operations, about to 15 minute contact in the oxidizer tank provides high conversion at 60-90 C., which temperature represents the normal range of white liquor before use for pulp manufacturing.

The following examples illustrate the eifect of sulfidity in this temperature range.

EXAMPLE 1 Sulfide concentration effect A sodium polysulfide content that can be produced is dictated by the starting Na s concentration; the reaction will work at Na S concentrations all the way from low to very high, with more polysulfide being formed as the starting Na S increases. This is shown in Table H for the oxidation of liquors of constant active alkali (106 g.p.l. Na O) and increasing sulfidity (increasing Na s) with Mn0 and 30 minutes at 60 C.

Effect of sulfidity and alkali content on conversion of sodium sulfide to sodium polysulfide The formation of sodium polysulfide by this oxidation depends only on the starting sulfide and not on sulfidity or alkali. This is shown in Table III in which the temperature of conversion was 70 C.

TABLE III Percent Initial Polysulfide Initial NazS, g.p.l. Sulfidity NaOH, g.p.l. Sulfur, g.p.l.

EXAMPLE 3 Temperature effect The formation of sodium polysulfide by oxidation of white liquor with manganese dioxide will take place over a wide temperature range as shown in Table IV for the oxidation of 30% sulfidity liquor of 40 g.p.l. Na s (95.6 g.p.l. NaOH) with /1 mole of MnO per mole of Na S.

TABLE IV Polysulfide Minutes Sulfur, g.p.l.

Final NaaS, g.p.l.

Temperature, C.

EXAMPLE 4 Use of manganese oxidant with black or white liquor or mixtures The oxidation of sodium sulfide at 70 C. with manganese dioxide takes place in black liquor or a mixture of 30% sulfidity white liquor and black liquor, as shown in Table V for oxidation with mole Mn0 per mole of Na s.

TABLE V Initial Final Nags, NazS, Polysulfide Liquor g.p.l. g.p.1. Sulfur, g.p.l.

5. 5 0. 73 Do 8.1 2.51 Mixture black and white (1:1) 11. 0 1. 65

Similar beneficial results are achieved with green liquor as shown in Table VI below. The oxidation of sulfide with /1 mole manganese dioxide is at a temperature of 70 C. in green liquor and is shown in Table VI for varying sulfide concentration at constant total alkali and in Table VII for constant sulfide concentration at varying total alkali.

Final Concentrations, Initial Concentrations, g.p.l.

g.p.l.

Total Alkali, Polysulfide N 323 N 32003 g.p.l. NazO Sulfur NazS EXAMPLE 5 Examples of various manganese oxidants at level of mole of manganese component per mole of Na S in kraft liquor.

As this example illustrates, in manganese dioxide, the prefered material, and other manganese oxides, hydroxides or sulfides employed, 30% and 100% sulfiidity white liquors (40 g.p.l. initial Na S concentration) are treated with various manganese oxidants at the 75% molar addition rate, as shown in Table VIII below.

TABLE VIII Percent Polysulfide N as Oxidant Additions 0. Min. Sulfidity Sulfur, g.p.l. g.p.l.

M11203 90 60 30 10. 95 6. 2 M11 100 3. 84 9. 1 M11304. 30 13. 23 6. 7 M11304" 100 11. 52 4. 1 MnSz. 30 4. 41 23. 8 Mn 100 3. 20 23. 4 Partially Oxidized Mn 30 1. 6 33. 3 Partially Oxidized MnO- 100 1. 4 15. 7 M OOH 30 12. 75 9. 5 100 7. 2 13. 4 30 10. 4 8. 3 100 6. 6 15. 8

EXAMPLE 6 Regeneration and reuse This example illustrates regeneration of the spent oxidant by air oxidation and reusing the same.

A 30% sulfidity white liquor of 40 g.p.l. Na s concentration was treated with excess manganese dioxide at 90 C. After separation of the polysulfide liquor, the oxidant was regenerated by aeration in dilute alkali, and then reused in another oxidation. This was repeated seven times with 30% sulfidity white liquor (40 g.p.l. Na S) with the results as shown in Table IX. The loses of oxidant are negligible.

Intermittent oxidation Table X below illustrates that the reaction can be stopped at any desired point to give varying concentrations of Na S and sodium polysulfide and also demonstrates that high efliciency is obtained for any degree of oxidation (conversion being defined as the moles polysulfide sulfur formed divided by the moles sodium sulfide oxidized, ex-

pressed as a percentage). This is shown for two sulfidity levels, 30 and 60%, at constant active alkali (varying initial Na S). The temperature was 70 C.

TABLE X Percent Final Polysulfide Initial NaOH, a; Na s, Sul r, Conversion Na s, g.p.l. g.p.l. Oxidized g.p.l. g.p.l. Percent EXAMPLE 8 Effect of proportions of manganese oxidant The following example shows the effect of proportions, percent moles of MnO per mole of Na S in oxidation of 40 g.p.l. initial Na S concentration at a temperature of C. The data are summarized in Table XI below.

TABLE XI Polysulfide Time in Percent M1102 Sulfur, g.p.l. Minutes EXAMPLE 9 Paper pulp production of increased yield and matching Kappa value with varying content of sodium polysulfide The kraft pulping which is carried out with this mixture is comparable to the conventional mixture of sodium sulfide and sodium hydroxide in a White liquor having a. sulfidity of at least 15% and up to 35%, the sulfidity being defined as the ratio of the sodium sulfide to the active alkali (sodium sulfide plus sodium hydroxide, both chemicals on Na O basis):

Nags

White liquors of 40 g.p.l. initials Na s concentration are treated with various manganese oxidants at the 75 addition rate, as shown in Table VIII.

In accordance with the present invention and as pointed out in Example 1, the sodium polysulfide that can be produced is dictated only by the starting sulfide concentration. This starting sulfide also dictates the amount of polysulfide that can be applied to the wood on a percentage basis and, thereby, the magnitude of the yield increase realized in pulping. Comparative batch digester data are shown in Table XII for a kraft cook and cooks made with an MnO oxidized white liquor. Sulfidity was adjusted to two levels, 30% and In all cooks, a 20 lb. charge (oven dried basis) of slash pine chips was employed with a 4.511 liquid-to-wood ratio. A cooking schedule of 45 minutes at C., 30 minutes to 173 C. and 90 minutes at 173 C. was followed in a two cubic foot digester with forced circulation and indirect heating.

It is seen from the above that an increase in yield of 2.3% was obtained on pulping with an oxidized 30% sulfidity liquor and 5.3% on pulping with an oxidized 100% sulfidity liquor at the same or approximately the same Kappa value.

EXAMPLE 10 Pulping with liquor prepared from regenerated oxidant The oxidizing power of a manganese oxidant can be exhausted through oxidation of sulfide and then regenerated by air oxidation to such a state that it is again capable of forming a polysulfide liquor that is effective in increasing pulp yield.

Hot white liquor was continuously passed through a bed of manganese dioxide until polysulfide was no longer produced. The oxidant was then regenerated by aeration in aqeuous alkali. The regenerated oxidant was then used to oxidize the sulfide in a fresh 30% sulfidity white liquor (40 g.p.l. Na s) to polysulfide pulping liquor. The polysulfide pulping liquor was then used for pulping experiments on slash pine chips. Comparative kraft pulps were prepared with a 30% sulfidity white liquor and the same cooking schedule. Pulp yields were calculated and Kappa numbers determined by Tappi standard T236m-60. The yield increase with the oxidized liquor is evident in the table below.

In the manganese oxidation process, a mill liquor is fed continuously along with manganese dioxide into conventional apparatus in which the liquor and oxidant are fed into a reactor. The reactor is of a size which will give suflicient retention time to provide the required degree of oxidation by the manganese oxidant at a uniform oxidant addition rate and a liquor addition rate at the desired temperature of reactions. The liquor slurry is continuously withdrawn from the reactor into a mechanical separator. The mechanical separator preferably comprises a centrifuge and a filter. Either may be used alone. The oxidized and clarified liquor can then be used for pulping. The insoluble and thickened material from the separator may be oxidized as such by air or first diluted somewhat with aqueous alkali and then oxidized by air in a regerr erator. It would then be thickened in another separator before reuse in treatment of additional starting liquor.

Iclaim:

1. The method of pulping wood material which comprises the steps of:

(a) providing a solution of sodium sulfide in an alkaline digesting liquor said sodium sulfide varying from 13 to 225 grams per liter;

(b) chemically oxidizing said sodium sulfide by the introduction of an aqueous insoluble, inorganic manganese oxidizer to produce sodium polysulfide and an aqueous insoluble spent manganese oxidant capable of being regenerated with oxygen-containing gas;

(c) separating said spent oxidant from said polysultide; and,

(d) pulping said wood with said polysulfide.

2. The method as defined in claim 1, wherein said spent oxidant is regenerated with an oxygen-containing gas.

3. The method as defined in claim 2, wherein said alkaline digesting liquor contains alkali in the form of sodium hydroxide, the sulfidity of the pulping liquor being from about 30% to about 4. The method as claimed in claim 2, wherein said insoluble solid inorganic manganese oxidizer is selected from the group consisting of Mn O Mn O MnS partially oxidized MnO, MnOOH and MnO- and said chemical oxidation is carried out at a temperature in the range of 40120 C.

5. The method as claimed in claim 2, wherein said alkaline digesting liquor contains sodium carbonate.

References Cited UNITED STATES PATENTS 2,944,928 7/1960 Kibrick et a1. 162-32 3,210,235 10/1965 Ferrigan et al. 16230 3,216,887 11/1965 Landmark 162-38 S. LEON BASHORE, Primary Examiner R. H. TUSHIN, Assistant Examiner US. Cl. X. R. 

